The present invention relates generally to the testing of semiconductor chips, and specifically to the design of an interconnect mechanism for use in probe card assemblies.
Typically, semiconductor chips are tested to verify that they function appropriately and reliably. This is often done when the semiconductor chips are still in wafer form, that is, before they are diced from the wafer and packaged. This allows the simultaneous testing of many semiconductor chips at a single time, creating considerable advantages in cost and process time compared to testing individual chips once they are packaged. If chips are found to be defective, they may be discarded when the chips are diced from the wafer, and only the reliable chips are packaged. They may also be tested after dicing but before packaging by assembling die on tape or a mechanical carrier.
Generally, modern microfabricated (termed MEMS) probe card assemblies for testing semiconductors have at least three components: a printed circuit board (PCB), a substrate to which thousands of probe contactors are coupled (this substrate hereinafter will be referred to as the “probe contactor substrate”), and a connector (often called an “interposer”) which electrically interconnects the individual electrical contacts of the PCB to the corresponding electrical contacts on the probe contactor substrate which relays signals to the individual probe contactors. The combination of the probe contactor substrate and the coupled contactors is sometimes referred to as the Probe Head. The probe contactors on the probe contactor substrate often have a very fine pitch (distance between the contactors) (30 um to 200 um) while the electrical contacts of the PCB and the interposer often have coarser pitches (>200 um). Thus, in modern MEMS probe card assemblies, the probe contactor substrate is often referred to as a space transformer as the substrate spreads out the electrical connections from the fine pitch of the electrical connections of the probe contactors to the coarser pitch found on the interposer and PCB. It should be noted that some probe cards do not utilize an interposer, but the general idea is unchanged. In most applications, the PCB and the probe head must be roughly parallel and in close proximity, and the required number of interconnects contained in the space transformer may be in the thousands or tens of thousands. The vertical space between the PCB and the probe contactor substrate is generally constrained to a few millimeters by the customary design of the probe card assembly and the associated semiconductor test equipment. It is also important that the contact tips of the probe head lie essentially in a plane. The background of U.S. Pat. No. 7,180,316, titled “Probe Head with Machined Mounting Pads and Method of Forming Same,” assigned to Touchdown Technologies, Inc. of Baldwin Park, Calif., discusses the importance of the planarity of the probe tips.
There are several ways of manufacturing a finished probe card assembly. A typical approach is shown in FIG. 12. This involves a series of critical time intensive steps which include, designing the electrical interconnect routing for the space transformer, designing the layout for the probe contactors (which may be lithographically fabricated), fabricating the space transformer, and then building or assembling the probe contactors onto the space transformer. However, this serial approach greatly increases the time required to manufacture a finished probe card assembly. Manufacturing the space transformer and the probe contactors are typically the most time consuming steps which is several weeks. A process whereby the manufacture of the probe contactors onto a substrate and the manufacture of the space transformer may be done simultaneously would result in significant savings of both time and cost.
Additionally, in the current form of manufacturing MEMS formed probe contactors, the space transformer substrate upon which the probe contactors are formed is an LTCC (Low-Temperature Co-fired Ceramic), HTCC (High-Temperature Co-fired Ceramic ), or thin-film high-density interconnect (HDI) substrate that allows for easy fabrication of vias (interconnections that allow electrical signals to pass from the top of the substrate to the bottom of the substrate) while redistributing the interconnections from the fine pitch necessary for the probe contactors to the coarser pitch needed by the interposer and PCB. These substrates are commonly used because they provide a compromise between providing the redistribution capability needed of the space transformer on the one hand and the mechanical strength, thermal properties, and construction elements needed for MEMS fabrication in the final assembly on the other hand.
However, LTCC/HTCC substrates are not the ideal substrates for MEMS manufacturing. They neither offer the optimum surface quality nor the optimum strength required for forming MEMS probe contactors on a substrate. To overcome these issues, certain undesirable features must be included in the LTCC/HTCC substrates. The issue of strength is usually overcome by using very thick LTCC/HTCC substrates and the issue of surface quality is overcome by further modifying the substrate with lapping/polishing or with coatings such as polyimide. All of these steps add complexity and time to the fabrication process. Additionally, substrates other than LTCC/HTCC are better suited for via redistribution, the primary function of the space transformer, however they lack the strength, thermal properties, or compatibility for MEMS manufacturing. In other words, LTCC/HTCC or thin-film HDI substrates are a compromise material for probe card substrates and space transformers. Thus, what is needed is a manufacturing process that will allow for the simultaneous, or parallel, fabrication of the probe contactor substrate and the space transformer, and that further allows for greater choice in selecting the material that forms the probe contactor substrate and the space transformer substrate.